Bonded magnets are manufactured from mixtures (compounds) of magnetic powders and binding resins (organic binders), by pressure-molding the mixtures into desired magnet shapes. Among such bonded magnets, a special type of bonded magnet referred to as a rare earth bonded magnet is made up from a magnet powder which is composed of a magnetic material containing a rare earth element or elements. Methods of manufacturing rare earth bonded magnets are disclosed, for example, in Japanese Patent Publication No. 53-34640, Japanese Patent Publication No. 46-31841, Japanese Patent Publication No. 04-74421, Japanese Patent Laid-Open No. 59-136907, Japanese Patent Laid-Open No. 59-213104, Japanese Patent Laid-Open No. 02-153509, Japanese Patent Laid-Open No. 60-211908, Japanese Patent Laid-Open No. 60-216523, Japanese Patent Laid-Open No. 61-164215, Japanese Patent Laid-Open No. 59-103309 and Japanese Patent Laid-Open No. 03-108301.
The methods of manufacturing rare earth bonded magnets are broadly sorted into compaction molding, injection molding and extrusion molding.
In compaction molding, the aforesaid compound is packed in a press mold and compacted at a room temperature so as to form a green body. Subsequently, when the binding resin is a thermosetting resin, the resin is hardened, whereby a magnet is obtained. This method enables the molding to be carried out with smaller amount of binding resin than other methods, resulting in a smaller resin content in the product magnet, thus advantageously contributing to improvement in the magnetic characteristics of the magnet.
Extrusion molding is a method in which heated molten compound extruded from an extruder die is solidified by cooling and then cut at a desired length, whereby a magnet is obtained. This method in one hand offers an advantage in that it permits easy production of thin-walled or elongated magnet by virtue of a comparatively large molding versatility on the shape of product magnet, but on the other hand suffers from a problem in that it requires, in order to ensure a sufficiently high fluidity of the molten compound during the molding, a greater amount of binding resin to be used as compared with the compaction molding method, with the result that the magnetic characteristics are impaired due to increased resin content in the product magnet.
In injection molding, the aforesaid compound, which has been heated and molten to exhibit sufficiently high fluidity, is poured into a mold so as to form a magnet of a desired shape. This method offers molding versatility on the magnet shape even greater than that offered by the extrusion molding method, enabling easy fabrication of magnets having irregular configurations. However, this method requires higher level of fluidity of the molten compound and, hence, a greater content of the binder resin than required in the extrusion molding method, resulting in poor magnetic characteristics of the product magnet due to increased content of the binder resin in the product magnet.
Among these methods, the compaction molding method enables production of magnets having superior magnetic performance as compared with other methods. Manufacture of bonded magnets by the known compaction molding method, however, suffers from the following disadvantages.
Firstly, it is to be pointed out that rare earth bonded magnets manufactured by this method tend to exhibit high porosity, which reduces mechanical strength and corrosion resistance of the product magnet. Hitherto, therefore, countermeasures have been taken in the compaction molding, such as use of high-pressure molding technique which employs a compaction pressure as high as 70 kgf/mm.sup.2 and anti-corrosion coating on the molded product. Elevated compaction pressure, however, heavily burdens the mold and molding machine, which in turn requires a greater dimensions of the mold and molding machine, incurring a rise of the production costs. In addition, the anti-corrosion coating does not achieve sufficient improvement in the resistance to corrosion.
A second problem is as follows. The compound is pelletized before subjected to the molding. It is often difficult, however, to smoothly charge the pellets of the compound into the mold and to completely fill up the mold cavity. In addition, pelletized compound does not permit delicate control of the rate of supply of the compound into the mold.
A third problem is as follows. When compaction molding is conducted on a compound containing a thermosetting resin, the compaction is effected at room temperature, regardless of whether the thermosetting resin is of the type which is in solid phase at the room temperature or of the type which is in liquid phase at the room temperature. Therefore, when the former type of thermosetting resin, i.e., the solid-phase resin, is used, moldability of the material is impaired, tending to exhibit greater porosity than that obtained when a thermoplastic resin is used. In addition, mechanical strength also tends to be reduced due to inferior dispersibility of the resin and the magnet powder. When the later-mentioned resin, i.e., the liquid-phase resin, is used, physical properties of the resin tends to be sensitively varied in accordance with the molding environment, e.g., temperature and humidity, often resulting in inferior charging of the mold, although a green body of high density is obtainable.
The second and third problems mentioned above cause the dimensions of the product magnets to substantially fluctuate from the target dimensions. Namely, the dimensional precision is impaired and the molding cannot be conducted at high degree of stability. These deficiencies are serious particularly when small-sized magnets are to be manufactured.
In order to obtain a product magnet in conformity with the target dimensions despite the substantial fluctuation in the dimensions, it is necessary that the molded article has dimensions greater than the target dimensions and that such molded article is subjected to a secondary processing such as milling or grinding into final shape and dimensions. Such a secondary work increases the number of steps of the manufacturing process, and increases the risk of production of unacceptable products, with the results that the production efficiency is lowered and the cost of production is raised.
The present inventors have discovered that one of the causes of the first to third problems described above is impropriety of factors such as method and conditions of preparation of the compound, molding conditions such as temperature, and post-molding conditions such as cooling condition.
Accordingly, an object of the present invention is to provide a rare earth bonded magnet which has a low porosity and which excels in moldability, mechanical and magnetic characteristics and dimensional stability, and to provide also a method, as well as a composition for rare earth bonded magnet, which enables easy manufacture of such a rare earth bonded magnet.